The present invention relates to improvements in a carrier lifetime measuring apparatus which measures a minority carrier (current carrier) lifetime within a semiconductor non-contactingly and nondestructively on the basis of the frequency dependence of a photovoltage.
It is well known that the minority carrier lifetime of a semiconductor is an important factor pertinent to the electrical characteristics of a semiconductor device. Therefore, the non-contacting and nondestructive inspection of the minority carrier lifetime is important in the enhancement of yield percentage in a manufacturing process.
The inventors have previously proposed a carrier lifetime measuring apparatus which measures minority carrier lifetime within a semiconductor nondestructively from the bending point of the frequency dependence of a photovoltage (Japanese Patent Application No. 57-87944). In the proposed apparatus, a p-Si wafer having a p-n junction, for example, is irradiated with radiation which has energy equivalent to or somewhat greater than the band gap of Si (in the case of Si, infrared radiation at wavelengths of 1-1.15 .mu.m) and which is chopped, to generate an alternating photovoltage, and this photovoltage V.sub.ph is detected through capacitance coupling which employs a transparent electrode, whereby the frequency dependence of the photovoltage stated above is measured. Thus, the apparatus makes it possible to nondestructively measure the minority carrier lifetime within the sample wafer without forming any electrode on the sample wafer.
The lifetime .tau. of carriers at the irradiated position of the sample wafer is obtained as .tau.=1/(2.pi.f.sub.0) from the frequency f.sub.0 of that bending point based on the carrier lifetime which appears in the frequency dependence of the photovoltage when this frequency dependence is measured. The photovoltage V.sub.ph becomes a constant value at frequencies lower than the cutoff frequency f.sub.c of the junction. In contrast, it varies in proportion of f.sup.-1 for frequencies exceeding f.sub.c (point A in FIG. 1A), and it varies in proportion to f.sup.-3/2 for frequencies exceeding f.sub.0 (point B in the figure). Accordingly, when the frequency dependence of the photovoltage V.sub.ph is indicated by coordinates both the axes of which are logarithmically represented, as shown in FIG. 1A, the frequency at the time at which the characteristic curve changes from a gradient of 45 degrees to a gradient of 56 degrees is found as the frequency f.sub.0.
The proposed apparatus, however, still involves problems as described below. Although the bending point of 45 degrees.fwdarw.56 degrees is a property often observed in cases of the p-n junction etc., the manifestation of the influence of the time constants of interface states or traps or surface states or traps is often observed in, for example, p-type Si bearing an oxide film or n-type Si subjected to an alkali surface treatment. The frequency dependence of the photovoltage in such case is no longer ensured to become a curve decreasing at the inclination angle of 45 degrees for f.gtoreq.f.sub.c, as seen from FIG. 1B. In addition, the appearance of a new bending point is sometimes observed. Accordingly, it is difficult to obtain f.sub.0 as to such samples, and there is the problem that the carrier lifetime .tau. cannot be measured from the bending point of the frequency dependence. Further, changes in an interelectrode capacitance, the input impedance of a photovoltage detector, etc. affect the frequency dependence of the photovoltage. Therefore, whether or not the observed bending point is truly based on the minority carrier lifetime needs to be confirmed by, for example, the photoconductivity decay method which requires the formation of electrodes. This signifies the problem that, with only the bending point of the frequency dependence of the photovoltage, the measured results lacks reliability.